Effect of straw returning on soil nitrogen transformation

The amount of soil nitrogen net mineralization in field cropped with winter wheat and summer maize was measured by in situ incubation method The soil microbial biomass nitrogen was measured by chloroform fumigation extraction and ninhydrin reactive nitrogen method The soil inorganic N was measured by continued flow analysis instrument In the soil covered with winter wheat straw in summer and incorporated with summer maize straw in autumn, the net N mineralization was 210kg/hm2 in the first year, 179 kg/hm2 in the second year The net mineralization was approximately equal to the amount of applied N In the non-straw returning soil the net mineralization was 164 and 248 kg/hm2 in the first and second year, respectively Incorporating maize straw to the soil resulted in increase of inorganic N during wheat season and positive effect of soil N mineralization Surfacing wheat straw in summer gave no significant difference in soil inorganic N The soil microbial biomass N in straw returning soil was 72～234% greater than that in the non-straw returning at beginning of each season, and was 34～72% greater at ending of each season The soil microbial biomass N in straw returning soil alternate during the first 2 years and did not be stable at a new level